The present invention relates to a transmission apparatus for use in a master station in a passive double-star communication network such as a passive optical network (PON) employing the asynchronous transfer mode (ATM).
ATM-PON technology has been undergoing recent research and development as a way to introduce optical fibers into subscriber lines in telecommunication networks.
FIG. 1 is a block diagram illustrating the basic ATM-PON connection topology. A plurality of slave stations 1-1 to 1-N, each having subscriber terminal equipment, are coupled to a master station 2 through a star coupler 3. The star coupler 3 is coupled to the master station 2 by an optical fiber 4, and to the slave stations 1-1 to 1-N by further optical fibers 5-1 to 5-N. ATM cells are transferred at arbitrary timings in the downstream direction, from the master station 2 to a slave station 1-n (where n is a number from 1 to N), and at timings specified by the master station 2 in the upstream direction, from slave station 1-n to master station 2. The upstream transmission timings specified by the master station 2 are spaced at intervals enabling the ATM cells from the slave stations to arrive at the master station 2 one after another, without mutual interference or collision.
The slave stations 1-1 to 1-N are located at different distances from the master station 2, however. If each slave station transmits cells at the specified timings, without further adjustment, ATM cells arriving at the master station 2 from different slave stations may partially overlap, because of different propagation delays.
The master station 2 accordingly measures the propagation delay of each slave station 1-n and reports the result of the measurement to slave station 1-n, which adjusts its ATM cell transmission timing accordingly. This adjustment is also referred to as a phase adjustment, because it causes the ATM cells from slave stations 1-1 to 1-N to arrive at the master station 2 in phase with one another on the same time axis. The entire process of measuring delays and adjusting upstream transmission timings is referred to as delay control.
A conventional delay control method was disclosed in 1993 in Japanese Unexamined Patent Application No. 5-336143.
In this method, the master station 2 sends a first phase correction time, a slave-station identification code, and a request signal requesting the transmission of a phase adjustment signal by the slave station in an idle time slot in the downstream direction. A time slot normally includes both ATM cells and physical layer operations, administration, and maintenance cells or PLOAM cells. An idle time slot, which is not used for communication, includes a PLOAM cell and a plurality of idle cells. If the received slave-station identification code matches the identification code of slave station 1-n, slave station 1-n sends a PLOAM cell in the upstream direction, delaying the upstream transmission timing by the first phase correction time. From the time of arrival of the received PLOAM cell and the first phase correction time, the master station 2 calculates the round-trip delay time of slave-station 1-n, calculates a second phase correction time that makes the round-trip delay equal to the round-trip delay of the most distant slave station, and sends the second phase correction time to slave station 1-n, again using an idle time slot. Thereafter, in sending ATM cells and other cells to the master station 2, slave station 1-n adjusts the transmission timing by the second phase correction time.
A problem in this conventional delay control method is that the master station 2 must already know the slave-station identification codes assigned to the slave stations 1-1 to 1-N. This places a major burden on network maintenance personnel. When a new slave station is added, for example, network maintenance personnel must speedily enter its slave-station identification code at the main station. It is also necessary for subscribers to report the slave-station identification code to network maintenance personnel each time a slave station is installed.
Another problem is the extensive use of idle time slots. When the round-trip delay time of slave station 1-n is being measured, an idle time slot is required in the upstream direction, so that the PLOAM cell sent by slave station 1-n does not collide with an ATM cell sent by another slave station. In the conventional method described above, to obtain this idle time slot in the upstream direction, the corresponding time slot in the downstream direction must also be left idle.
In data communication, bandwidth usage in the upstream and downstream directions is often asymmetrical. The network could be managed more efficiently if it were possible to send ATM cells with user data in the downstream direction even during an idle time slot in the upstream direction.
It is accordingly an object of the present invention to provide a master-station communication apparatus that can measure delay times without the need to acquire slave-station identification codes in advance, and can guarantee efficient network operation.
According to a first aspect of the invention, a transmission apparatus for use in a master station in a passive double-star communication network comprises a management unit and a measurement unit. The management unit automatically acquires information identifying slave stations, by having the slave stations transmit cells to the master station. The measurement unit measures the propagation delays of the slave stations after their identifying information has been acquired.
According to a second aspect of the invention, the management unit manages control states related to delay control in the transmission apparatus as a whole, while the measurement unit manages the state of the delay-measurement operation for each slave station individually. The control states managed by the management unit include a delay-measurement control state. In response to certain events, the management unit sends a command specifying one of the slave stations to the measurement unit, then enters the delay-measurement control state.